The invention relates to a device comprising an electron-emitting body and means for coating an electron-emitting surface of the body with a layer of a material reducing the electron work function of the surface, the body and means located in an envelope which is evacuated or filled with an inert protective gas. Coating is accomplished by means of a decomposition reaction of a suitable material or by heating a mixture, in which the material reducing the electron work function is released and deposited on the electron-emitting surface.
The electron-emitting body may be a thermionic cathode or a semiconductor cathode; in the latter case, various kinds of semiconductor cathodes may be used, such as NEA cathodes, field emitters and especially reverse-biased junction cathodes as described in U.S. Pat. Nos. 4,303,930 and 4,370,797, assigned to the present Assignee. Vacuum tubes containing such cathodes tubes are suitable to be used as camera or display tubes, but may also be used in apparatus for Auger spectroscopy, electron microscopy and electron lithography.
The relevant device may also be provided with a photocathode, in which event incident radiation gives rise to an electron current which leaves the photocathode. Such photocathodes are used in photocells, camera tubes, images converters and photomultiplier tubes.
Another application of a device according to the invention is the so-called thermionic converter, in which thermal radiation is converted into an electron current.
An inert protective gas is to be understood herein to mean a gas which does not adversely influence the decomposition reaction which occurs, for example, upon heating the mixture. The quantity of protective gas present in the envelope can be slightly varied under the influence of the reaction, in which the material reducing the work function is released, as will appear below.
The invention further relates to a method of applying a thin layer of a material reducing the electron work function of an electron-emitting surface of an electron-emitting body in an evacuated space or a space filled with an inert protective gas, the material reducing the electron-work function being obtained by a decomposition reaction or heating of a suitable mixture.
Such a method is known from Netherlands Patent Specification No. 18,162. In this case, cesium is deposited in a discharge tube by heating a dissolved mixture of cesium chloride and barium oxide so that the cesium chloride is reduced by the released barium to metallic cesium, which spreads over the interior of the discharge tube. In an embodiment shown in the said Patent Specification, the mixture to be heated is provided in a lateral branch of the vacuum tube which is sealed from this take afterward.
Although mention is made in the said Patent Specification of the possibility to provide the mixture at areas in the discharge tube other than in a lateral tube, there is no indication about the manner in which this could be achieved.
A possible solution is to heat cesium chromate together with a reduction agent (silicon or zirconium) on a resistance tape of tantalum in the vacuum by passing a current through the said resistance tape, which leads to the desired heating. In practice, however, a number of problems then arise.
Firstly, problems arise due to the use of tantalum as resistance material for heating purposes. In order to obtain a sufficient power for the reduction of the cesium chromate (about 1 to 2 W), it is required in practice that electric currents of a few Amperes are passed through the resistance tape. In a number of applications, for example Auger spectroscopy, electron microscopy and electron lithography, in which substantially all elements are operated at a high voltage, this often means that an additional transformer is required. The current moreover has to be passed to the resistance tape via supply wires and lead-through pins; in view of the high currents, these lead-through pins have a diameter of 0.5 to 1 mm. The disadvantage of such thick lead-through pins in vacuum tubes is generally known.
Disadvantages also arise from the use of cesium chromate and the reduction reaction to which it is subjected. This reaction cannot easily be controlled and may sometimes even lead to an explosion. From this reaction, moreover, a considerable number of by-products, such as water vapour (H.sub.2 O), carbon dioxide (CO.sub.2) and cesium oxide (Cs.sub.2 O) are obtained. The comparatively high temperature at which the reaction takes place (about 725.degree. C.) not only gives rise to the said high power required to heat the resistance tape, but also results in an unfavourable ratio between the quantity of pure cesium and, for example, cesium oxide in the released gas mixture. The ratio of the vapour pressure of pure cesium to that of cesium oxide in fact rapidly decreases with increasing temperature. A possible solution to this problem is the removal of residual products via overdistillation by pumping and allowing released cesium to be deposited on a cooling surface, after which it is spread again by careful heating. However, this solution comprises a number of steps (such as cooling, for example by a Peltier element, and heating again), which are preferably avoided in high-vacuum, high-voltage applications.
The invention has for its object to provide a device of the kind mentioned in the opening paragraph, in which the said problems are substantially avoided.
In addition, the invention has for its object to provide a method in which an electron-emitting surface can be coated in a controlled manner with a layer of material reducing the electron work function of the surface.